Transcript profiling technology enables quantitative measurement of the transcriptional activity of potentially thousands of genes in biological samples. The application of such technology to toxicology, toxicogenomics, promises substantial dividends in mechanistic toxicity research and also, possibly, the ability to predict adverse toxicity for novel or untested compounds. Our laboratory has developed a custom approach to this technology, designing cDNA microarray platforms specifically for gene expression events of relevance to a large number of toxicological endpoints. Such arrays allow comprehensive coverage of genes associated with entire pathways (such as oxidative stress, signal transduction, stress response, epithelial biology) and enable simultaneous measurement of more than ten thousand gene expression events. (C) 2001 Published by Elsevier Science Ireland Ltd.